This application is a 371 of PCT JP 98/05284 filed Nov. 24, 1998.
The present invention relates to a method of preparing pentafluoroethane, catalysts for fluorination and a preparation method thereof.
Generally, since saturated halogenated hydrocarbons containing hydrogen (hereinafter sometimes referred to as alternative flon) have an extremely low possibility of destroying the ozone layer, they have attracted much attention as an alternative to halogenated carbons without hydrogen (for example, chlorofluoroethanes: hereinafter sometimes referred to as specific flon) which have been used on the market.
In particular, pentafluoroethane is expected to satisfy a wide range of uses as a refrigerant, a foaming agent, a solvent, and a dry etchant. Further, pentafluoroethane is a useful substance because it is an inert and low-toxicity gas under normal temperatures and pressures.
In the conventional preparation methods for pentafluoroethane, it has been known that a gas phase fluorination reaction of tetrachloroethylene or halogenated hydrocarbons [C2HClXF(5-x); wherein x is 1-5] uses a chromium oxide catalyst or an alumina catalyst carrying metals and so on.
However, it has been obvious that the conventional known reaction cannot avoid the generation of chlorofluoroethanes (specific flon) as by-products, in addition to the objective product, pentafluoroethane and halogenated hydrocarbons represented as C2HClxF(5-x) (where x is 1-5) which can be recycled in the reaction system as the starting material.
In addition, chlorofluoroethanes [C2ClXF(6-x); where x is 1-5] of this specific flon cannot be recycled in the reaction system as the starting material, resulting in a production loss, difficulty in separation from the objective product (pentafluoroethane) in the purification process leading to a cost increase for the purification equipment and lower purification, especially in the case of 1-chloro-1, 1, 2, 2, 2,-pentafluoroethane (hereinafter referred to as CFC-115). Further, much expense is required for the treatment.
First, the conventional methods of preparing pentafluoroethane using a chromium oxide catalyst or an activated carbon catalyst carrying chromium or chromium oxide are described.
For example, it is disclosed in U.S. Pat. No. 3,755,477 that pentafluoroethane is produced using 2,2-dichloro-1,1,1-trifluoroethane (hereinafter referred to as HCFC-123) as a starting material in the presence of a chromium oxide catalyst. JP Open. No. 8-38904 illustrates that pentafluoroethane is generated from perchloroethylene in the presence of a chromium oxide catalyst treated reductively. Further, in WO No. 92/19576, it is disclosed that pentafluoroethane is produced from HCFC-123 in the presence of a chromium oxide prepared from (NH4)2Cr2O7. It is also disclosed in EP No.456552 that pentafluoroethane is produced using HCFC-123 as a starting material in the presence of an activated carbon catalyst carrying Cr.
However, since the conventional methods using said catalysts, such as a chromium oxide catalyst and an activated carbon catalyst carrying chromium or chromium oxide, are limited in reducing the problematic chlorofluoroethane by-products as described above, improvement in the problem of generating chlorofluoroethanes is still insufficient.
Especially in any of the said methods, it is difficult to lower the ratio of the total yield of chlorofluoroethane by-products to not more than 1% of the yield of the objective product, pentafluoroethane.
Another method of preparing pentafluoroethane is known using alumina or aluminum fluoride catalysts carrying metals.
For example, the methods of preparing pentafluoroethane are disclosed using starting materials such as perchloroethylene, 1,1,2-trichloro-2,2-difluoroethane (hereinafter referred to as HCFC-122) or HCFC-123 in the presence of the catalysts: Cr2O3/Al F3 in EP No. 638535; Mn (or Co, Cr)/AlF3 in JP Publ. No. 3-505328; Zn/alumina in WO No. 92/16482; Co/Ce/alumina in JP Open. No. 4-29940; and Cr/Ni/Al oxide catalyst in EP No. 609124.
Compared to the cases using chromium oxide catalysts, the cases using these catalyst with alumina or aluminum fluoride as a carrier lead to a low reactivity being forced to react at a high temperature. Consequently, it not only creates conditions for an increase in generation of the by-products, but also leads to increase both in equipment costs for heat of reactors and running costs. Further, the larger reactor made of a higher quality material is required resulting from the necessity for a large amount of the catalyst.
New catalysts for the preparation of pentafluoroethane are proposed in which the catalyst is a chromium catalyst such as chromium oxide or fluorinated chromium oxide which has been made to carry metals.
However, in most of these, the yield of the chlorofluoroethane by-products is not considered. In addition, they do not illustrate the preparation methods of pentafluoroethane showing a highly-active fluorination; namely, a high yield of pentafluoroethane and a controlled generation of chlorofluoroethane by-products.
For example, in JP Open. No. 2-178237, pentafluoroethane is prepared by fluorination of perchloroethylene using a catalyst Fe2O3xe2x80x94Cr2O3with a good yield. In JP Open. No. 7-61944, pentafluoroethane is obtained by using HCFC-123 as a starting material and a catalyst In/ CrOxFy (the catalyst shown in the published patent gazette is specified with a composition that gives a Cr valence of +3.0) treated with hydrogen at 400xc2x0 C. for 4 hr. In JP Open. No. 8-108073, preparation of pentafluoroethane is also disclosed using HCFC-123 as a starting material and a catalyst Ga/CrOxFy (the catalyst shown in the published patent gazette is specified with a composition that gives a Cr valence of +3.0) treated with hydrogen at 400xc2x0 C. for 4 hr. Both gazettes show the controlled effect on lowering the activity due to an increase in the reaction pressure and the long life of the catalyst.
However, none of the said examples show findings relating to the generation of chlorofluoroethane by-products.
Namely, although WO No. 95/27688 discloses a method of preparing pentafluoroethane using catalysts of Zn/Cr oxides, this does not illustrate the finding relating to the total yield of chlorofluoroethane by-products. The amount of CFC-115 is shown, however, in the example using perchloroethylene as a starting material, the ratio of CFC-115 to pentafluoroethane generated is 0.59%. Further, in the example using HCFC-123 as a starting material, even the ratio of CFC-115 to the combined amount of HFC-125 with HCFC-124 already reaches 1.46%; accordingly, the reducing effect on chlorofluoroethanes is insufficient.
Further, a method of preparing pentafluoroethane from perchloroethylene using an Mg/Cr oxide catalyst is disclosed in EP No. 733611. However, large amounts of chlorofluoroethane by-product are obviously generated in all examples shown there, compared with the comparative example, and the ratio of the total amount of chlorofluoroethane generated to the amount of pentafluoroethane generated is high, and in the range of 2.9 to 7.0%.
In another fluorination reaction of 2-chloro-1,1,1-trifluoroethane (hereinafter referred to as HCFC-133a), a chromium oxide catalyst with the addition of some metals is also proposed.
For example, the fluorination reaction of HCFC-133a is disclosed in JP Open. No.2-172933 using a catalyst comprised of halogenated compounds or oxides containing Cr and at least one element selected from a group composed of Al, Mg, Ca, Ba, Sr, Fe, Ni, Co, and Mn. The fluorination reaction of HCFC-133a is also disclosed in EP No. 546883, where the catalyst composed of a base material of an oxide of Cr mixed with Ni prepared by a hydroxide sol of Cr3+with Ni2+is used.
However, this literature does not show findings relating to activity and selectivity of the generation reaction of pentafluoroethane or the yield of chlorofluoroethanes. Also, the reactivity of the generation reaction of pentafluoroethane can not be easily predicted. Further, since the catalyst in the former is calcinated at 450xc2x0 C. for 5 hr and also the catalyst in the latter is prepared through calcination at 420xc2x0 C. for 4 hr, the average valence of the Cr becomes approximately +3.0. Therefore, these conditions are unsuitable to obtain an amorphous catalyst.
The present invention has been accomplished in consideration of the existing facts described above. The purpose is to provide a method of preparing pentafluoroethane using catalysts that are capable of (1) reducing the total yield of chlorofluoroethane by-products when preparing pentafluoroethane without significantly deteriorating the generation activity of the objective product, pentafluoroethane, and C2HClxF(5-x) where x is an integer between 1 and 5), which can be recycled in the reaction system as a starting material; (2) consequently controlling production losses as well as purification equipment costs; and (3) improving the purity of the pentafluoroethane produced.
Another object of the present invention is to provide catalysts for fluorination which can be used in the preparation of pentafluoroethane described above and a preparation method thereof.
In order to solve said problems, the present inventors have examined improvements to chromium catalysts such as chromium oxide and fluorochromium oxide and found that chromium catalysts in an amorphous state with an average valence of the chromium not less than +3.5 but not more than +5.0 and with the addition of at least one metal element selected from a group composed of indium, gallium, cobalt, nickel, zinc and aluminum are capable of reducing the total yield of chlorofluoroethane by-products when preparing pentafluoroethane without significantly deteriorating the generating activity of pentafluoroethane and of C2HClxF(5-x) (where X is an integer between 1 and 5), which can be recycled in the reaction system as the starting material.
Namely, as a result of an examination aiming at the relationship between the chromium valence and the reactivity in the added chromium catalyst, the present inventors have revealed that said effect is obtained using catalysts for which the average valence is about +4. This is assuming that the valence variation to be considered as one of the catalytic properties of the chromium is easy to occur. Because a valence of +6 is thermally unstable and also often shows sublimation, it has sometimes posed problems in use as a catalyst.
Further, it is proved that the average valence of chromium as an entire catalyst specified by the results of the composition analysis and determination of the magnetic susceptibility does not always have a comparatively stable integer value of +3, +4 or +6. That is because the average valence is not considered to give integer values due to mixing of chromiums of valence of +3, +4 or +6,
For the above reason, the average valence of the chromium, including mainly chromium with a valence of +4, should be defined as not less than +3.5 but not more than +5 (preferably +4 to +4.5). By being not less than +3.5, the catalytic activity is improved compared with lower values, and by being not more than +5.0, the catalyst becomes highly active with a stable structure compared with higher value. Further, by adding said metals to these chromium catalysts, a highly-active catalyst which generates hardly any chlorofluoroethanes is obtained with the above effect.
Namely, the present invention relates to a method of preparing pentafluoroethane (hereinafter referred to as the present inventive method of preparing pentafluoroethane) wherein chlorine-containing carbon compounds are fluorinated under the presence of chromium catalysts that are in an amorphous state. The main component of said chromium catalysts is chromium compounds with the addition of at least one metal element selected from the group composed of indium, gallium, cobalt, nickel, zinc and aluminum. The average valence of chromium in said chromium compounds is to be not less than +3.5 but not more than +5.0.
As aforementioned, although some chromium oxide or fluorinated chromium oxide catalysts having a chromium valence of +3 with added metals are proposed, the following findings have been found through meticulously repeated studies by the present inventors:
In the presence of a chromium catalyst in an amorphous state which contains the chromium compound with the addition of at least one metal element selected from the group composed of indium, gallium, cobalt, nickel, zinc and aluminum, this compound having a valence larger than+3, in which the compound is stable, and smaller than +6, in which the compound often shows sublimation, and preferably not less than +3.5 but not more than +5.0;
by fluorinating such compounds as chlorine-containing hydrocarbons, including perchloroethylene, 1,1-dichloro-2,2,2-trifluoroethane and 1-chloro-1,2,2,2-tetrafluoroethane, both a highly-active pentafluoroethane generation reaction and a decrease in the ratio of the yield of chlorofluoroethane by-products to the yield of pentafluoroethane can be brought about.
In particular, said high activity leads to a decrease in both equipment costs and running costs by enabling a lower reaction temperature, decrease in the catalytic amount, and long-life catalyst. Moreover, since in practice hardly any chlorofluoroethane by-products are generated, further generation of chlorofluoroethane by-products can be controlled.
The activity in the main reaction (the generation reaction of pentafluoroethane) of the catalyst in an amorphous state with a chromium valence of +3.5 to +5 is much higher than the activity of a crystalline catalyst or a catalyst with a chromium valence of +3. This high activity, which is a characteristic of a catalyst""s amorphous nature, can be fully utilized to control the amount of specific flon generated by adding metal elements. In addition, to generate the same amount of HFC-125, the high activation of the amorphous catalyst enables the lowering of the reaction temperature and, due to the effect of the added metals, results in a dramatically lower absolute amount of specific flon than if using a crystalline catalyst.
Herein, said average valence of the present invention means a chromium valence specified by composition analysis and determination of the magnetic susceptibility. The average valence of the chromium in said catalyst specified by said composition analysis is calculated from the result obtained by actually conducting a composition analysis on the said chromium catalyst. The average valence of the chromium in said catalyst specified by determination of the magnetic susceptibility is theoretically calculated from the result obtained by the change in the magnetic susceptibility of said catalyst resulting from a change in temperature. (concrete measurement methods will be described later.)
Herein, said average valence is preferably not less than +3.6 but not more than +4.8 (furthermore preferably not less than +4.0 but not more than +4.5).
Further, said amorphous state means an amorphous state of the whole catalyst and this is the state where no diffraction peak assigned to a specific crystalline structure exists in the X-ray diffraction measurement, for example.
The fluorination reaction of the present inventive method of preparing pentafluoroethane may use HF, F2 or the other fluorine-containing hydrocarbons as the fluorinating agent.
The present invention provides chromium catalysts for fluorination which can be used in the present inventive method of preparing pentafluoroethane. The main component of said chromium catalysts, which is in an amorphous state (hereinafter referred to as the present inventive catalysts), is chromium compounds with the addition of at least one metal element selected from the group composed of indium, gallium, cobalt, nickel, zinc and aluminum. The average valence of the chromium in said chromium compounds is not less than +3.5 but not more than +5.0.
The present inventive catalyst is a chromium catalyst for fluorination reaction submitted to various fluorination reactions, preferably in particular submitted to a method of preparing pentafluoroethane obtained by fluorination of ethanic and ethylenic chlorinated hydrocarbons (or chlorinated carbons).
Further, as a preparation method of the present inventive catalyst with good reproducibility, when preparing the chromium catalyst in an amorphous state for fluorination, the main component of which is chromium compounds with the addition of at least one metal element selected from the group composed of indium, gallium, cobalt, nickel, zinc and aluminum and the average valence of the chromium in said chromium compounds being not less than +3.5 but not more than +5.0;
the present invention provides a method of preparing catalysts for fluorination (hereinafter referred to as the method of preparing the present inventive catalyst) wherein the chromium catalyst in said amorphous state for fluorination is obtained by calcinating in an atmosphere of inert gas.
According to the method of preparing the present inventive catalyst, when preparing the present inventive catalyst, the chromium catalyst for fluorination which is in said amorphous state can be obtained by calcinating in an atmosphere of inert gas such as nitrogen, for example. Said calcination can be conducted at a temperature of 380xc2x0 C. to 410xc2x0 C. for 0.5 hr to 3.5 hr.
First, the present inventive method of preparing pentafluoroethane is illustrated.
In the present inventive method of preparing pentafluoroethane, it is preferable to fluorinate at least one of said chlorine-containing hydrocarbons selected from the group composed of perchloroethylene, 1,1-dichloro-2,2,2-trifluoroethane and 1-chloro-1,2,2,2-tetrafluoroethane by hydrogen fluoride.
Further, it is desirable that said chlorine-containing carbon compounds are any of 1,1-dichloro-2,2,2-trifluoroethane (HCFC-123) and 1-chloro-1,2,2,2-tetrafluoroethane (HCFC-124) or their mixture.
Namely, when using HCFC-123 and HCFC-124 (especially HCFC-124) as the starting material, the generation of chlorofluoroethane by-products can be controlled to an extremely small amount and simultaneously the objective pentafluoroethane can be obtained with a high yield.
Of course, perchloroethylene can be used as the starting material and in this case, it is considered that perchloroethylene is converted to pentafluoroethane through intermediates such as HCFC-123 and HCFC-124.
The present inventive method of preparing pentafluoroethane as described above is extremely effective especially in the reaction which generates chlorofluoroethane by-products. Accordingly, in the present invention, such a reaction is not limited to starting materials. However, the reaction is extremely effective in the case of fluorination by hydrogen fluoride of any of perchloroethylene, 1,1-dichloro-2,2,2-trifluoroethane and 1-chloro-1,2,2,2-tetrafluoroethane or their mixture or a mixture of 1,1-dichloro-2,2,2-trifluoroethane and 1-chloro-1,2,2,2-tetrafluoroethane for which the yield of pentafluoroethane is relatively high and the amount of chlorofluoroethanes generated is large. 1,1,2-trichloro-2,2-difluoroethane (HCFC-122) and the like may be used as the said chlorine-containing carbon compounds.
The conditions of the fluorination reaction in the present inventive method of preparing pentafluoroethane can be selected according to the characteristics of each starting material.
For example, the molar ratio for the reaction of HF with the chlorine-containing carbon compounds (especially any of perchloroethylene, HCFC-123 and -124) as starting materials is usually (0.9-100):1, and the reaction temperature is usually 150xc2x0 C. to 450xc2x0 C. The contact time of said catalyst with the reaction gas (value obtained by dividing the catalyst weight by the amount of reaction gas flowing per unit time) is usually 0.1 g/NmLxc2x7sec-50 g/NmLxc2x7sec. For this, preferable reaction pressure can be selected appropriately by the species of starting gas submitted to the reaction.
For example, in the fluorination reaction which generates pentafluoroethane from HCFC-123, the conversion ratio to pentafluoroethane and the yield of chlorofluoroethanes can be varied by appropriately changing the molar ratio, the reaction temperature, the contact time, and the pressure of reaction of the fluorinating agents such as HF with HCFC-123. The conversion ratio to pentafluoroethane tends to rise as the reaction temperature and the contact time increase.
Considering the cost of manufacturing equipment, the running costs for the reaction conditions and the above reaction conditions, the reaction temperature should preferably be 250xc2x0 C. to 380xc2x0 C. and the molar ratio of the reaction should be preferably 2 to 10. Further, the contact time should preferably be 0.2 g/NmLxc2x7sec-20 g/NmLxc2x7sec. Furthermore, the reaction pressure should preferably be near atmospheric pressure. Although the reaction can be conducted under pressures higher than atmospheric pressure, the conversion ratio to pentafluoroethane tends to lower.
According to the present inventive method of preparing pentafluoroethane, when fluorinating 1,1-dichloro-2,2,2-trifluoroethane as said chlorine-containing carbon compound under the presence of said chromium catalyst, the ratio of the total yield of chlorofluoroethane by-products to the yield of pentafluoroethane obtained can be fully controlled to be not more than 0.5%.
Further, when fluorinating 1-chloro-1,2,2,2-tetrafluoroethane as said chlorine-containing carbon compound, the ratio of the total yield of chlorofluoroethane by-products to the yield of pentafluoroethane obtained can be fully controlled to not more than 0.3%.
Thus, according to the present inventive method of preparing pentafluoroethane, the efficient preparation of pentafluoroethane can be conducted without significantly deteriorating the yield of pentafluoroethane and generation activity of C2HClXF(5-x) (where x is an integer between 1 and 5), which can be recycled, and the ratio of the total yield of chlorofluoroethane by-products to the yield of pentafluoroethane obtained can be controlled to not more than 0.5% or 0.3%, and in addition, the total yield of chlorofluoroethane which can not be recycled to the reaction system as a starting material is lowered, and the generation of CFC-115 which is difficult to separate from pentafluoroethane can be controlled.
In the present inventive method of preparing pentafluoroethane, said chromium compounds may be at least one species selected from the group composed of chromium oxide, chromium fluoride, fluorochromium oxide and chlorofluorochromium oxide. Namely, the catalyst used in the present inventive method of preparing pentafluoroethane may be mixtures of such various forms, for example, fluorochromium oxide and the like including chromium of different valences.
Prior fluorination is preferable before said chromium catalyst is submitted to the fluorination reaction of said chlorine-containing carbon compounds. For example, the fluorination can be conducted by setting said catalyst at a prescribed temperature and for a prescribed time in a mixture of HF gas and N2 gas.
Herein, said chromium catalyst such that the specific surface area becomes 25-130 m2/g after said fluorination can be used. Namely, chromium oxide including amorphous Cr of high valence with added metals is prepared by the above method, thereafter the catalyst of 25-130 m2/g is spontaneously obtained by performing HF gas treatment at 100 to 460xc2x0 C., preferably at 150 to 400xc2x0 C. This reaction is conducted using HF diluted with nitrogen gas (HF: 5 to 20%) at a low temperature (150 to 250xc2x0 C.) in the beginning of the HF treatment which is an extremely large exothermic reaction; thereafter a rise in the temperature or increase in the HF concentration is desirable.
The amorphous state of said chromium catalyst can be made by calcinating said chromium catalyst conducted in an atmosphere of inert gas.
The amorphous chromium catalyst having the valence described above can be formed by said calcination within the above temperature and time. Especially, said calcination is preferably conducted within the temperature and time ranges of 380 to 410xc2x0 C. and 0.5 to 3.5 hr, respectively.
Moreover, in the present inventive method of preparing pentafluoroethane, addition methods of said metal elements to said chromium catalyst are not limited. For example, after immersing chromium hydroxide in an aqueous solution of said metal elements followed by drying, said calcination may be conducted (impregnation method or immersion method: refer to FIG. 1). Or after obtaining chromium hydr-oxide containing said metal elements by coprecipitation from an aqueous solution dissolving said metal elements and chromium followed by drying, said calcination may be conducted (coprecipitation method: refer to FIG. 2).
Namely, for example, there is the impregnation method wherein chromium catalysts or chromium hydroxide, which is the precursor of the chromium catalyst, are immersed in an aqueous solution of said metal salts followed by drying and calcination, and there is the coprecipitation method wherein a precipitant such as aqueous ammonia is added to a mixed aqueous solution comprising an aqueous solution of metal salts of said metal elements and an aqueous solution of chromium salt to give chromium hydroxide containing said metal elements and is followed by calcination. Using these methods, at least one metal element selected from the group composed of In, Ga, Co, Ni, Zn and Al is added to a chromium compound.
While the added amount of said metal element to the chromium catalyst such as chromium oxide may not be too small to obtain the present inventive effect, it may not be enough to significantly inhibit the reactivity of the chromium catalyst. Namely, the ratio of number of atom to Cr in the catalyst is desirable within the range of 0.001 to 0.5, and preferably 0.005 to 0.1 (the method of preparing the present inventive catalyst is the same).
Thus, when fluorinating said catalyst by HF and the like in order to stabilize the initial activity of said catalyst and to dehydrate, addition of said metal elements is possible after said fluorination treatment; however, it is desirable to be conducted before fluorination treatment.
In the present inventive method of preparing pentafluoroethane, the addition or carrying of an element having an improving effect on the reactivity or selectivity to said chromium catalyst except chromium, indium, gallium, cobalt, nickel, zinc, aluminum, oxygen, chlorine and fluorine, is desirable. Said element may be at least one element selected from the group composed of cadmium, magnesium and titanium.
When fluorinating said chlorine-containing carbon compounds using hydrogen fluoride, a part or all of the generating products can be returned to the reaction system or can be led to another reaction system in which fluorination is conducted by hydrogen fluoride using catalysts as claimed in claim 1. Further, separating the mixture containing pentafluoroethane and hydrogen chloride from said products, the residual products may be returned to said reaction system or may be led to said another reaction system.
Such processes make it possible to prepare pentafluoroethane more efficiently.
Next, the present inventive catalyst is illustrated.
In the present inventive catalysts, said chromium compounds may be at least one species selected from the group composed of chromium oxide, chromium, fluoride, fluorochromium oxide and chlorofluorochromium oxide. Namely, if the chromium valence is within the range of said valence number, it may be of said various forms.
The present inventive catalyst is desirably fluorinated. Especially, the specific surface area after said fluorination is preferably within 25-130 m2/g.
In the present inventive catalysts, the addition of an element having an improving effect on reactivity or selectivity except chromium, indium, gallium, cobalt, nickel, zinc, aluminum, oxygen, chlorine or fluorine is desirable. Said element may be at least one element selected from the group composed of cadmium, magnesium and titanium.
Next, the method of preparing the present inventive catalyst is illustrated.
By treating chromium hydroxide obtained from a preparation method described later at high temperature in an inert atmosphere, dehydration as well as generation of chromium oxide proceed, and simultaneously, the surface area increases. These phenomena proceed by calcination even under the conditions of a comparatively lower temperature of between 300xc2x0 C. and 370xc2x0 C., or a short time of 0.4 hr and a sufficiently large surface area is obtained. However, many hydroxyl groups (OHxe2x88x92) in the catalyst remain in this calcination condition and thus the chromium valence in the catalyst obtained becomes a value below +3.5. Inversely, in calcination conditions of above 410xc2x0 C. or beyond 3.5 hr, the valence of the chromium ion transfers from stable +4 to +3. On the crystal structure of chromium oxide, the catalyst also transfers from an amorphous to a stable Cr2O3 (Cr +3 valence) crystalline structure and in addition the average valence of chromium becomes a value below +3.5.
From the above examination results, in order to obtain a catalyst where the average valence of chromium in the amorphous state shows not less than +3.5 but not more than +5.0 by calcinating chromium hydroxide obtained from the preparation method illustrated in the present application, calcination at 380xc2x0 C. to 410xc2x0 C., especially 400xc2x0 C. and for 0.5 to 3.5 hr, preferably 2 hr in an inert atmosphere is suitable.
After immersing powdered chromium hydroxide in an aqueous solution of said metal elements followed by drying, said calcination can be conducted (impregnation method or immersion method: refer to FIG. 1). Or after obtaining chromium hydroxide containing said metal elements by coprecipitation from an aqueous solution dissolving said metal elements and chromium followed by drying, said calcination maybe conducted (coprecipitation method: refer to FIG. 2).
Said chromium compounds in the method of preparing the present inventive catalysts may be at least one species selected from the group composed of chromium oxide, chromium fluoride, fluorochromium oxide and chlorofluorochromium oxide.
In the method of preparing the present inventive catalyst, fluorination after said calcination of said catalyst is desirable. The said fluorination can be conducted for controlling the specific surface area of said catalyst for fluorination to within 25-130 m2/g.
Further, in said catalyst for fluorination in the present inventive method, addition of an element having an improving effect on reactivity or selectivity except chromium, indium, gallium, cobalt, nickel, zinc, aluminum, oxygen, chlorine and fluorine is desirable.
Said element may be at least one element selected from the group composed of cadmium, magnesium and titanium.
In the method of preparing the present inventive catalyst, the average valence number of the chromium in said catalyst is not less than +3.5 but not more than +5.0 and the catalyst is amorphous. Such a catalyst for fluorination is prepared by the following method, for example.
First, precipitate of chromium hydroxide is obtained by mixing an aqueous solution of chromium salt (for example, chromium nitrate, chromium chloride, chrome alum and chromium sulfate) and aqueous ammonia.
Next, for example, precipitate of chromium hydroxide is obtained by dropping 10% aqueous ammonia equivalent to about 1.2 times into a 5.7% aqueous solution of chromium nitrate. In this process, although the property of chromium hydroxide can be controlled through the reaction rate of the precipitation reaction, the reaction rate is desirably rapid to a considerable extent. This reaction rate is controlled by the temperature of the reaction solution, the mixing method for the aqueous ammonia (mixing rate) and the stirring conditions.
Subsequently, this precipitate is filtered, washed and dried. This drying is desirably performed at 70xc2x0 C. to 140xc2x0 C. in air, especially at around 120xc2x0 C. for 1 hr to 50 hr, especially for around 12 hr, for example. The material obtained in this step is chromium hydroxide.
Next, after this chromium hydroxide is immersed, for example, in an aqueous solution of indium nitrate (for example, in order to control the atomic ratio of indium in the aqueous solution of indium nitrate to chromium in this chromium hydroxide to be about 1:0.03, the concentration of said aqueous solution is adjusted) for about 12 hr, chromium hydroxide with added indium is obtained by drying at 120xc2x0 C. for 12 hr, for example.
Next, to make a pellet-shaped catalyst, after grinding the chromium hydroxide with added indium, the shape is molded into a pellet by a tableting machine. The pellet may be cylindrical of 3 mm in diameter and about 3 mm in height. Considering the pressure loss and the diffusion of the gas flow, said pellet is desirably molded in a hollow cylindrical form.
By calcinating the molded catalyst in an atmosphere of an inert gas such as nitrogen, the average valence number of the chromium is not less than +3.5 but not more than +5.0 and the amorphous chromium oxide is prepared.
The calcination temperature is preferably not less than 380xc2x0 C. However, Cr2O3 (chromium valence is +3) composition is formed if too high; therefore, it is desirable to set the higher temperature within the range that is capable of avoiding it. Accordingly, this calcination is carried out, for example, at 380xc2x0 C. to 410xc2x0 C., especially around 400xc2x0 C. for 0.5 hr to 3.5 hr, especially for around 2 hr. As described above, if this calcination time is longer than the above range, Cr2O3 is generated and the chromium catalyst having a high average valence number cannot be obtained, while if too short, the number of the remaining hydroxyl group in the catalyst tends to become too high.
Next, fluorination treatment of the catalyst can be carried out with CFCs, HCFCs, HF or F2. If the fluorination treatment is not conducted, the generation reaction of objective pentafluoroethane is severely inhibited and generation of by-products is sometimes accelerated since fluorination of the catalyst proceeds during the generation reaction of pentafluoroethane.
Especially in the case of fluorination treatment with HF, the higher the temperature and the pressure are, the larger the rate of progress is. The temperature at which the generating water is not condensed in the process and at which the catalyst is not crystallized by the reaction heat may be maintained as the upper limit. For example, the temperature when fluorinating may be within the range of 100xc2x0 C. to 460xc2x0 C.
Herein, the specific surface area of the catalyst decreases in the range of 25 m2/g to 130 m2/g by this fluorination treatment.
Except for the above method, preparation of the chromium catalyst is possible wherein the average valence number of chromium in the catalyst is within not less than +3.5 but not more than +5.0 and the catalyst is in the amorphous state. However, in order to obtain the present inventive catalyst, the various conditions for obtaining chromium hydroxide by precipitation reaction (neutralization reaction) and calcination condition of chromium hydroxide and so on in the above method of preparing the catalyst are especially significant.
Herein, as for an example of another preparation method except the above preparation examples of the catalyst, there are coprecipitation methods for obtaining the precipitate of chromium hydroxide by mixing an aqueous solution of indium nitrate with an aqueous solution of chromium nitrate instead of impregnating an aqueous solution of indium nitrate into chromium hydroxide.
According to the present inventive method of preparing pentafluoroethane, since (1) chlorine-containing carbon compounds are fluorinated in the presence of chromium catalysts that are in an amorphous state; (2) the main component of said chromium catalysts is a chromium compound with the addition of at least one metal element selected from the group composed of indium, gallium, cobalt, nickel, zinc and aluminum; and (3) the average valence of chromium in said chromium compound is not less than +3.5 but not more than +5.0, it is possible to decrease the total yield of chlorofluoroethane by-products when preparing pentafluoroethane without significantly deteriorating the generation activity of the pentafluoroethane and C2HClxF(5-x) (where x is an integer between 1 and 5), which can be recycled in the reaction system as a starting material.
The present inventive catalysts, which are in an amorphous state and wherein the main component is chromium compounds with the addition of at least one metal element selected from the group composed of indium, gallium, cobalt, nickel, zinc and aluminum and the average valence of chromium in said chromium compounds is not less than +3.5 but not more than+5.0, are useful as catalysts for fluorination in various fluorination reactions and can be submitted especially to the present inventive method of preparing pentafluoroethane.
According to the method of preparing the present inventive catalyst, when preparing the chromium catalyst in an amorphous state for fluorination wherein chromium compounds with the addition of at least one metal element selected from the group composed of indium, gallium, cobalt, nickel, zinc and aluminum are contained as the main component and the average valence of the chromium in said chromium compounds is not less than+3.5 but not more than+5.0, since the chromium catalyst in said amorphous state for fluorination is obtained by calcination in an atmosphere of inert gas, the chromium catalyst in said amorphous state for fluorination can be prepared with good reproducibility.